Coherent lightwave communication systems are continuing to be designed in the long wavelength regime from 1.2 .mu.m to 1.6 .mu.m using Group III-V semiconductor lasers to take advantage of optical fiber loss minima. Two main advantages to such systems are nearly ideal receiver sensitivity and improved frequency selectivity.
In coherent lightwave communication system applications, it is necessary to maintain reasonably close tolerances on wavelength (frequency) stability of the optical sources. (It is understood that, because of the mathematical relationships between "wavelength" and "frequency", these terms are used interchangeably herein without loss of generality.) That is, transmitter sources and receiver local oscillator sources must be synchronized to a common wavelength or to two separate wavelengths having a constant wavelength offset from each other. Unfortunately, semiconductor lasers exhibit sufficiently large frequency drifts due to junction temperature and injection current variations that their use as transmitter sources, receiver local oscillators, system frequency references, or reference clocks in such systems is made feasible only after the addition of complicated frequency control or compensation loops to stabilize and control the wavelength of the semiconductor laser.
Frequency stabilization techniques for semiconductor lasers have been widely discussed in the technical literature since the early part of the present decade. While the scope of techniques discussed is quite broad to include the use of Fabry-Perot interferometers and atomic or molecular transition lines as references, most reports are limited to the short wavelength region involving AlGaAs lasers which operate around 0.8 .mu.m. See, for example, C. J. Nielsen et al., J. Opt. Commun., Vol. 4, pp. 122-5 (1983) dealing with the use of Fabry-Perot interferometers; S. Yamaguchi et al., IEEE J. Quantum Electron. Vol. QE-19, pp. 1514-9 (1983) on the use of atomic transition lines; and H. Tsuchida et al., Jpn. J. Appl. Phys., Vol. 21, pp. L1-L3 (1982) on the use of molecular transition lines.
Fabry-Perot interferometric techniques are vulnerable to long term drifts caused by fluctuations of the resonant cavity. Longer term stability is afforded by using atomic or molecular transition lines. Of the latter approaches, atomic transition lines are preferred over molecular transition lines. Atomic spectra offer relatively few, widely separated and, therefore, easily identifiable strong lines. On the other hand, molecular transition line spectra are complex and weak which, in turn, results in the need for very long absorption cells as a reference.
Only a few articles have reported experiments for semiconductor laser frequency stabilization on longer wavelength semiconductor lasers operating above 1.2 .mu.m. Such sources are usually based on Group III-V materials such as InGaAsP/InP. The reported experiments solely employ molecular transition lines as references. In one experiment, absorption lines of the ammonia (NH.sub.3) molecule are used to frequency stabilize an InGaAsP distributed feedback laser. See, T. Yanagawa et al., Appl. Phys. Lett., Vol. 47, pp. 1036-8 (1985). Another experiment employed first overtone vibration-rotation lines of hydrogen fluoride (HF) molecules to frequency lock an InGaAsP laser. See, S. Yamaguchi et al., Appl. Phys. Lett., Vol. 41, pp. 1034-6 (1982). One other experiment utilized absorption lines of water vapor (H.sub.2 O) molecules and ammonia molecules in spectral measurements for pollutant gas monitoring. See M. Ohtsu et al., Jpn. J. Appl. Phys., Vol. 22, pp. 1553-7 (1983).
It is noteworthy that experiments reported in the long wavelength region are restricted to using only molecular transition lines. In part or in whole, this is apparently due to the fact that there has been much reported difficulty, and there have been no reported successes, in finding useful atomic transitions in this wavelength region emanating from the ground state. But, a careful review of the literature shows that one need not be restricted to only those frequency stabilization techniques which employ atomic transitions from the ground state. It is well known to use optogalvanic signals corresponding to transitions from excited atomic states for stabilizing short wavelength AlGaAs semiconductor lasers. See, for example, S. Yamaguchi et al., IEEE J. Quantum Electron., Vol. QE-19, 1514-9 (1983) describing the optogalvanic effect of krypton.
Unfortunately, extension of the optogalvanic effect to frequency stabilization techniques for longer wavelength semiconductor lasers was cast in serious doubt in 1982 when the recognized experts in the field stated that output power from longer wavelength semiconductor lasers (InGaAsP/InP) is insufficient to produce an impedance change in the discharge tube. Since the optogalvanic effect concerns large impedance changes of the gas discharges by optical (laser) irradiation at wavelengths corresponding to non-ionizing transitions of species present in the discharge, the experts deduced that the optogalvanic signals would be difficult, at best, to detect for the longer wavelength lasers. See, S. Yamaguchi et al., Appl. Phys. Lett., Vol. 41, pp. 1034 (1982). It is reasonable to surmise that the failure to have published works over the last score on the extension of the optogalvanic signal techniques for frequency stabilization developed and improved by yamaguchi et al. at short wavelengths is, in large part, due to the chilling effect on other researchers of the experts' own published statement which knowingly predicts failure of the technique at longer wavelengths.
When applied to lightwave communication systems, absolute frequency standards have been suggested for stabilizing semiconductor lasers in transmitters whereas frequency (IF) stabilization via feedback control circuits have been used for locking receiver local oscillators substantially to the transmitter frequency (i.e., at the transmitter frequency or slightly offset from that frequency). In known coherent lightwave communication systems, regardless of the stabilization method for the transmitter source, IF tracking and correction circuits at the receiver have been and are continuing to be used to stabilize each receiver local oscillator in synchronism with the frequency of the transmitter source. See, for example, Y. Yamamoto et al., IEEE J. of Quantum Electronics, Vol. QE-17, No. 6, pp. 919-935, 923 (1981) wherein absolute stabilization is employed at the transmitter laser and IF locking feedback control circuitry is employed for the receiver local oscillator laser.